Remodeling of cortical networks in response to visual experience during development relies on specific and localized changes in synaptic structure and function, the progression and mechanisms of which are still poorly understood. While intracellular signaling has been the focus of most studies of the visual cortex, the extracellular environment plays a crucial role in limiting visual plasticity. We hypothesize that extracellular pathways, which are believed to be restricted to the injured or diseased brain, are surprisingly also active during plasticity in the visual cortex and allow structural remodeling at single synapses. Using imaging techniques which allow real time visualization of cellular morphology in vivo, we can, for the first time, visualize the rewiring of cortical networks during manipulations of vision at the level of a single synapse and directly examine entirely novel mechanisms of this rewiring. Using these methods, we will test the following scenario: monocular deprivation leads to extracellular protease activity resulting in microglial activation and extracellular matrix degradation, which in turn implement structural and functional plasticity. In aim 1 we will quantify key aspects of this structural synaptic plasticity and determine its synaptic locus. In aim 2, we will assay microglial involvement in visual plasticity and in aim 3 we will elucidate the extracellular pathway that leads to microglial activation and extracellular matrix remodeling. We will use a combination of in vivo functional and structural 4D imaging with histological, molecular and genetic tools which will provide unique insights into the extracellular mechanisms underlying plasticity in the visual cortex. Because of the broad range of morphological and functional synaptic deficits observed in neurodevelopmental and neurodegenerative disorders, accomplishing these aims will also provide insights into new treatment avenues for such diseases. PUBLIC HEALTH RELEVANCE: The structure of synapses is crucial to the proper functioning of the nervous system: changes in this structure are observed during brain development, learning and in many neurological disorders, including autism, epilepsy and Alzheimer's disease. In this proposal we will study how synapse structure is modulated by natural stimuli in the intact brain and identify mechanisms mediating synaptic changes. This study will provide information with broad implications for understanding and treating a large spectrum of human neurological disorders.